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Crawtord Myron L/EMR lest facilities ev QC100 U56 NO.87-3080 1987 V19 C.1 NBS-P

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PUBLICATIONS

OO U56 NO.87-3080 1987 V19 C.1 | |

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EVALUATION OF REVERBERATING CHAMBER

LOCATED AT RADC, GRIFFISS AFB

ROME, NEW YORK

Myron L. Crawford

Galen H. Koepke

John M. Ladbury

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303-3328

December 1987

-0C

100

. U56

#87-3080

1987

Research Information Center

National l»ureau of SLauoards

Gaithersburg, Maryland 20399

NBSIR 87-3080

EMR TEST FACILITIES

EVALUATION OF REVERBERATING CHAMBER

LOCATED AT RADC, GRIFFISS AFB

ROME, NEW YORK

Myron L. Crawford

Galen H. Koepke

John M. Ladbury

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303-3328

December 1987

Sponsored by

Compatibility and Measurements Division

RADC/RBC

Griffiss AFB, New York 13441

U.S. DEPARTMENT OF COMMERCE. C. William Verity. Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

CONTENTS

Page

List of Figures and Tables v

1 . 0 Introduction

2.0 Description of the RADC Reverberat ing Chamber

Facility and MBS Evaluation Systems

3.0 Reverberat ing Chamber CW Evaluation Results

3.1 Antenna Coupling Efficiency and VSWR

3.2 Chamber Quality Factor

3.3 Tuner Effectiveness

3.4 Test Zone E-Field Uniformity

3.5 E-Field Amplitude Calibration

3.6 Comparison of RADC Reverberating Chamber

with MBS Anechoic and Reverberating Chambers

4.0 Rever berat i ng Chamber Pulsed RF Evaluation Results

4.1 Background

4.2 Evaluation of Pulsed Rf Measurement Results

5.0 Summary of CW Measurement Uncertainty

5.1 Estimate of Uncertainty in Establishing CW

E -Field Amplitude Inside the Chamber

5.2 General Comments

6.0 Summary and Conclusions

7.0 Acknowledgments

8.0 References

iii

LIST OF FIGURES AND TABLES

Page

F i gure ? . 1 Cross sectional views of RADC reverberating chamber snowing

placement of tuner, transmitting and receiving antennas, am:

orobe used to evaluate E field amplitude inside the chamber 21

Figure 2.2 Pnotographs of interior of RADC reverberating chamber showing:

(a) tuner mounted from ceiling and transmitting antenna mounted

on tripod in northeast corner of chamber, and (b) receiving

antenna mounted on tripod in northwest corner of chamber

Figure : . 3 Photograph showing NBS equipment used in performing chamber

evaluation 24

Figure 2.4 Block diagram of H3S system used in cw evaluation of RADC

reverberating chamber 25

Figure 2.5. Block diagram of system used for evaluating pulsed rf response

characteristics of RADC reverberating chamber

Figure 3.1 Statistical representat ion of the composite VSWR of the

transmitting antennas used to launch the fields inside the

RADC reverberating chamber

Figure 3.2 Coupling efficiency (minimum and average losses) between

transmitted and received powers measured at antenna's terminals

inside RADC reverberating chamber . Losses determined for

(a) cw measurements using mode -tuned, (b) cw measurements

using mode -stirred and (c) from Dulsed rf measurements

F igure 3.3 Theoretical composite Q and experimental Q' determined for - ' ' 1 reverberat ing chamber: (a) theoretical and experimental values

of Q as a function of frequency, (b) ratio of the theoretics:

composite Q to experimental Q* as a function of frequence

F igure 3.4 Examples of E-field inside the chamber as a function of tun-"”

position for 1 watt net input power: (a) 100 PR z, (b. 15 , and (c) ?00 MHz

F igure 3.5 Ratio of maximum to minimum received power obtain- d by r:

•

•

. •

• tuner in the frequency range 100 v 'Hz to 1 3 CHz inside threverberating chamber

F igure 3.5 Cross sectional views of RADC reverberating chamber showing

placement of NBS isotropic probes for evaluation of s:

‘ distribution of E-fields

F i gur e 3.7 Pnotographs of interior of RADC reverberat ing :na- placement of NBS isotropic probes for eval j -.ion •

• , distribution of E-fields

V
Figure 3.8 Spatial distribution of the E-field measured inside the R ADC

reverberating chamber using array of 10 MBS isotropic probes:

(a) average, and (b) maximum. Net input power normalized to

1 watt. Transmitting antennas are log periodic, 0.1 GHz to

1.0 GHz, and ridged horn, 1.0 GHz to 2.0 GHz 33

Figure 3*9 Average values of the E-field strength measured inside RADC

reverberat i ng chamber using array of 10 NBS isotropic probes

with 1 watt net input power: (a) average of the averages,

(b) average of the maximums. Transmitting antennas same as figure 3.8 35

Figure 3-10 Average and maximum E-field strengths determined inside RADC

reverberating chamber using: (1) reference antenna's received

power measurements (+ + + + ), and (2) calibrated 1 -cm dipole probe

measurements (****). Net input power normalized to 1 watt 36

Figure 3-11 Comparison of the peak responses of NBS 1 -cm dipole probe to

normalized E -field of 37 dB V/m determined using RADC

reverberat ing chamber and NBS anechoic chamber 37

Figure 3.12 Comparison of (1 12 GHz) ridged horn's peak responses to

normalized E-field of 37 dB V/m established inside RADC

reverberating chamber and inside NBS anechoic chamber:

(a) antenna output versus frequency, (b) Comparison of

responses with horn's free-space gain subtracted from

anechoic chamber results 38

Figure 3.13 Comparison of peak response of (1-18 GHz) and (1 -12 GHz) ridged

horn antennas to 37 dB V/m normalized E-fields established

inside RADC reverberating chamber 39

Figure 3.19 Average and maxumum E-field strengths inside empty NBS

reverberating chamber for 1 -W net input power determined from:

(a) reference antenna's received power measurements, and

(b) calibrated 1-cm dipole probe measurements 90

Figure 3-15 Comparison of 1-cm dipole probe's peak response to EM field

established inside NBS reverberating and anechoic chambers.

Output normalized to E-field exposure of 37 dB V/m 91

Figure 4.1 Maximum received rf pulse waveforms inside RADC reverberating

chamber determined with increasing amounts of rf absorber

loading at 0.9 GHz. Measurements were made using dual channel

digitizing oscilloscope 92

Figure 9.2 Maximum received rf pulse waveforms inside RADC reverberating

chamber with (a) no absorber, (b) 1 piece 5" x 29" x 24" rf

absorber and (c) 1 piece 24" x 24" x 24" rf absorber at

selected frequencies. Measurements were made using dual

channel digitizing oscilloscope 44
Figure 4.

Figure *1.

Figure 4.

Figure 4.

Figure 4.

Table 3.1

Table 4.1

Table 4.2

Table 5.1

Table 5.2

Table 5.3

3 Statistical amplitude distribution (maximum and average) for rf

pulse waveforms obtained inside RADC reverberat ing chamber at

selected frequencies, input pulse widths and absorber loadings.. 47

4 Time required for rf signal transmitted into RADC reverberating

chamber to rise 63% of steady state amplitude as a function of

frequency. Chamber empty (no absorber), with 1 piece of

5" x 24" x 24" rf absorber, and with 1 piece of 24 " x 24" x 24"

rf absorber 57

5 Time required for rf signal transmitted into RADC reverberat i ng

chamber to rise to 90% of steady state amplitude as a function

of frequency. Chamber empty, with 1 piece of 5" x 24" x 24" rf

absorber, and with 1 piece of 24" x 24" x 24" rf absorber 58

6 Estimated correction factor for amplitude response of test

signal inside RADC reverberating chamber as a function of rf

input pulse duration at selected frequencies

7 Estimated correction factor for amplitude response of test

signal inside RADC reverberating chamber as a function of

frequency at selected rf input pulse durations. (Data extracted

f rom figure 4.6)

Spatial variations in the E-field average and maximum values

measured inside the RADC reverberat ing chamber

Influence of absorber loading on loss, quality factor and

charge/decay time of RADC reverberating chamber at 0.9 GHz

Comparison of measured and calculated chamber charge/decay tim.es

determined from chamber average loss measurements and from c m. of figure 4.3. Chamber empty, with 1 piece 5" x 24" x 2^" ('

absorber, and with 1 piece 24" x 24" x 24" rf absorber

Summary and estimates of measurement uncertainties for

determining cw field strength inside RADC reverberating ch , Modi- Tuned (100 MHz - 2.0 GHz)

Summary and estimates of measurement uncertainties for

determining cw field strength inside RADC reverberat ing •

• t Mode-Stirred (1.0 GHz - 18.0 GHz)

Estimates of upper limit of impedance mismatch un t inties for

reference antenna's received power measurements for ' reverberating chamber

vii

Electromagnetic Radiation Test Facilities

Evaluation of Reverberating Chamber

Located at RADC , Griff iss AFB , Rome, New York

Myron L. Crawford

Galen H. Koepke

John M. Ladbury

Electromagnetic Fields Division

National Bureau of Standards

Boulder, Colorado 80303

This report describes measurement procedures and results obtained

from evaluating the reverberating chamber facility located at the

Rome Air Development Center (RADC), Griffiss Air Force Base,

Rome, New York. The facility was developed by the RADC for use

in measuring and analyzing the electromagnetic

susceptibility/vulnerability (EMS/V) of weapon systems and the

shielding effectiveness of enclosures and shielding materials. A brief description of the facility, including the instrumentation

used for performing its evaluation and calibration by the

National Bureau of Standards (NBS) , is given. Measurements

described include: (1) evaluation of the chamber's transmitting

and receiving antennas' voltage standing wave ratios;

measurement of the chamber's insertion loss or coupling

efficiency versus frequency; (3) measurement of the chamber's

tuner effectiveness; (4) determination of the E-field uniformity

in the chamber's test zones versus frequency; (5) determination

of the absolute amplitude calibration of the test E-fields in the

chamber based upon the reference antenna's received power

measurements and calibrated dipole probe antenna measurements;

(6) comparison of reference standard equipment under test (EY7

responses to test fields established inside the RADI

reverberating chamber and the NBS anechoic chamber ; and 7 evaluation of the performance characteristics of t r.

• r everberating chamber excited by pulsed rf at selected tisrret'

frequencies as a function of pulse width (0.2 - 20 ys) anc t

r chamber's quality factor (Q). Conclusions indicate that to

chamber can be used at frequencies down to approximate! y 1

5

r v

for cw testing and, for rf pulsed immunity testing with pi.

widths as short as 0.3 ys by using rf absorber loading,

testing to pulsed rf fields however has some inherent limitations

that are discussed in the report. Estimates of * measurement uncertainties derived empirically f ro -r to

results are given .

Key words: electromagnetic radiated susceptibil ity/vuln< rabi ity

measurements; reverberating chamber.

1
1 . 0 Introduction

The use of a reverberating chamber for performing EMS/V measurements is

relatively new. Considerable work has been done in the past to evaluate and

document methods for using this technique [1-H]. Recently, research work has

been done at the National Bureau of Standards (NBS) to carefully evaluate,

develop (when necessary), describe, and document the methodology for

performing radiated susceptibility/vulnerability (EMS/V) measurements using a reverberating chamber. This effort is described in an NBS publication, NBS

TN 1092, and IEEE Proceedings review paper [5,6]. In addition, preliminary

work was performed to evaluate the time domain response characteristics of a rever berat ing chamber with the purpose of determining the feasibility of

using such a chamber for pulsed rf immunity testing. This work was published

as a progress report to the sponsor [7] dated January 1987. Included as an appendix to the report was a paper, "Preliminary evaluation of a

r e ver ber at i on chamber method for pulsed rf immunity testing " [8], which

gives 'a brief discussion of the theoretical concepts pertinent to the effort.

The incentive for performing this work stems from numerous advantages

suggested for the use of a reverberating chamber. These include:

1. Electrical isolation from or to the external environment;

2. Accessibility (indoor test facility);

3. Ability to generate high level fields efficiently over large test

volumes

;

A. Broad frequency coverage;

5. Cost effectiveness;

6. Potential use for both radiated susceptibility and emission testing

with minor instrumentation changes;

7. No requirement of physical rotations of the equipment under test

(EUT); and

8. Security.

These advantages are somewhat offset by limitations, which include loss

of polarization and directivity information relative to the EMC/EMI profile

of the EUT and somewhat limited measurement accuracy. However, this

technique does offer a time-efficient, cost-effective way to evaluate the

EMS/V performance of large equipment using a shielded enclosure with minor

modifications. The measurement concept utilizes the shielded, high-Q,

multimoded environment to obtain uniform (time averaged) fields that may

simulate "real world", near-field environments. Also, it may well be the

only technique by which very high exposure fields can be safely generated for

performing EM susceptibility tests required by the Department of Defense for some of their "real world" applications.

These considerations, along with others, motivated RADC to invest in

the research and development of this methodology and finally to construct and

place into operation the facility whose description and evaluation are given

in this report. Measurements described were performed between January 26 and

February 1 6 , 1987.

2
2.0 Description of the R ADC Reverberat i ng Chamber Facility and NBS

Evaluation Systems

The RADC rever berat ing chamber is made from a large modular shielded

enclosure 3.69 m x 5.18 m x 9.78 m (12.11 ft x 17.03 ft x 32.09 ft) in size.

The enclosure is constructed from galvanized steel panels with an integrated

panel and joiner system. Cross sectional views of the chamber are shown in

figure 2.1. It is equipped with two access doors, a small personnel door

located adjoining the shielded instrumentation and operator control room and

a large double door located at the end of the chamber. The chamber is

equipped with a large tuner shown in figure 2.2 (a). The locations and

orientations of the chamber's excitation (transmitting) and reference

receiving antennas are shown in figures 2.2 (b) and 2.2 (c). The purpose of

these antenna placements (located in the corners of the chamber oriented

toward the corners) is to couple the transmitted signal into all possible

modes and to monitor the reference received signal from all possible modes as efficiently and uniformly as possible. This must be done without favoring

particular modes or transmitting directly into the chamber's test zone, or coupling the signal directly between the transmitting and receiving antennas.

This is necessary to obtain a statistically uniform, spatial distribution of

the field in the chamber's test zone.

A photograph and the block diagrams of the basic systems used to

evaluate the chamber are shown in figures 2.3, 2. A, and 2.5. Figure 2.3 is - photograph showing the NBS equipment used in performing the champ 0 - evaluation tests. Figure 2. A is the block diagram of the NBS system used r or the cw evaluation measurements and figure 2.5 is the block diagram of the ’S-S system used for the pulsed rf evaluation measurements of the RADC chamber.

The test field was established inside the chamber by means of an rf sou- • (cw or pulsed as appropriate for the tests being performed) connected to

appropriate transmitting antenna. Modes excited inside the chamber w • re • n

• stirred by rotating the tuner which functions as a f ield perturbinr lev:

The test zone in the chamber is defined as the chamber's volum- m : r. .s minimum separation from the walls and ceiling of approximately 1/; -v

•

• . This assures an ample mode density which is a necessary condition for '

• validity of the r ever ber at i ng chamber technique [6]. Placement of an should fall within this volume except, possibly, relative to the floor,

separation distance between the EUT and the floor may be less than 1/

depending upon the EUT intended use configuration relative to th ij plane. Test leads and cables were routed to appropriate moni'ors,

outside the chamber through shielded feed-through connectors : resistance lines. This was done to prevent leakage of the EM fi< 1

• outside environment or into the instrumentation room. A pr^ : . . attenuator was used whenever possible between the power detec* • ;c

analyzer and the receiving antenna. This was done to mini -i.

mismatch with the receiving antenna. For the cw tests,

bidirectional coupler was used at the input to the transmit' : • -• allow measurements of the net input power. For the pulsed rf t- t , incident signal was measured. For mode stirred measuremerh ,

• was measured only at the beginning of the me*surement cy . the measurement cycles proceed under computer control , an • recorded, managed, and processed for presentation ar • contain"'. . ,7 .

The chamber was evaluated using two different op-r •

.

• • • referred to as mode-tuned and mode-stirred

.

3
For the cw mode-tuned tests (100 MHz - 18 GHz), the tuner was stepped

at selected, uniform increments, permitting measurements of the net input

power supplied to the transmitting antenna, the receiving antenna power,

f ield -measuring probes responses and the EUT response at each tuner position.

This allowed corrections to be made for the changes in the transmitting

antenna's input impedance as a function of tuner position and frequency. The

measurement results were then normalized to a constant net input value (1

watt for these tests). The number of tuner steps used per revolution were

200 (increments of 1.8 deg).

For the cw mode-stirred tests (1 .0 - 18.0 GHz), the tuner was

continuously rotated while sampling the reference antenna received power,

field probe response and the EUT response at rates much faster than the tuner

revolution rate. These measurements were made using a spectrum analyzer,

diode detectors, and "smart" voltmeters that are capable of data storage and

calculation of statistical functions such as mean values and standard

deviations. The mode stirred approach allows large data samples (up to

9,999) to be obtained for a single tuner revolution. Tuner revolution rates

were adjusted to meet the EUT output monitor and diode probe response time

requirements . Typical rates used were approximately 3 to 6 minutes per

revolution.

The transmitting and reference receiving antennas used for the cw tests

were log periodics (0.1 - 1.0 GHz), and ridged horn antennas (1.0 - 18.0

GHz) .

For the pulsed rf tests, the mode-tuned approach was used to optimize

measurement accuracy, to obtain complete statistical information for

evaluating the time domain response char act er i s t i cs of the reference

antenna's received signal, and to determine total energy content in the

received pulse relative to the normalized energy in the transmitted pulse.

The mode-stirred approach was used for relative measurements with the

digitizing oscilloscope placed in its maximum-hold mode of operation. This

approach is much faster than the mode-tuned approach and allows much large

data sampling than the mode tuned approach; however, it is less accurate and

does not provide complete statistical results since only maximum values are

recorded. The approach used was determined by the final results needed. The

mode-tuned approach was required for complete statistical data and total

energy (transmitted verse received) analysis. The mode-stirred approach was used to analyze relative information such as charge and decay time and

chamber loss as a function of amounts of rf absorber placed in the chamber.

For the pulsed rf mode-tuned tests (0_. 9 - 1 6 . 0 GHz), the tuner was stepped at selected, uniform increments, permitting measurements of the

transmitting antenna's net input signals' and the reference antenna's

received signals' time-domain parameters at each tuner position. This

allowed corrections to be made for the changes in the transmitting antenna's

input impedance variations as a function of tuner position and frequency so that the results could be normalized to a constant net input value. (This is

similar to the approach used for the cw tests.) The wave form data were then

processed to determine the statistical parameters as a function of time. For

these measurements, 200 tuner steps per revolution were used (1.8 deg

increments )

.
For the pulsed rf mode-stirred tests, the tuner was continuously

stepped while sampling the maximum received signal at a rate much faster than

the tuner revolution rate. This approach allows large data samples to be

obtained for a single tuner revolution.

The transmitting and receiving antennas used for the pulsed rf

measurements were a matched pair of rectangular ridged-horns designed to

operate from 0.8 GHz to 18 GHz. The instrumentation used for measuring the

transmitted and received signals include calibrated diode detectors and a dual channel digitizing oscilloscope. One channel of the digitizer was used

to measure the transmitted rf pulse monitored on the sidearm of a calibrated

broadband directional coupler and the second channel was used to measure the

received signal. The digitizing oscilloscope is capable of measuring signals

with rise times in the order of 30 ps at a sampling rate of 50 kHz/s and

sample sizes up to 1024/scan.

3.0 Reverberating Chamber CW Evaluation Results

How well a shielded enclosure can be made to operate as a reverberat ing

chamber depends upon a number of interacting considerations. The major

requirement is that the enclosure be large compared to the wavelength of the

lowest frequency intended for use so that sufficient modes, necessary to

obtain statistical spatial field uniformity, exist. This should be true for

the RADC chamber described above at frequencies above approximately 100 MHz.

3.1 Antenna Coupling Efficiency and VSWR

The efficiency with which energy can be injected into or coupled out of

the chamber via the transmitting and receiving antennas is determined by:

(1) the voltage standing wave ratio (VSWR) of the antennas (that is the

impedance match between the rf source and the transmitting antenna or between

the receiving antenna and its output detector), and (2) the ability of the

antennas to couple energy into or out of the particular modes that exist at the test frequencies of interest. Rotating the tuner changes the chamber's

boundary conditions and hence shifts the mode excitation frequencies,

changes the char acteristics of the field inside the enclosure which in ‘

. r,r

. influences the effective VSWR of the antennas. Hence the net input pow^r •

j and the measured received power from the enclosure varies as a function f tuner position. (that is the impedance match between the antennas and tr> .• source or termination affects the power transfer between the two.)

result in errors in determining the amplitude of the field inside r • enclosure. Variations, determined statistically, in the VSWR of t r. transmitting antennas used to excite the RADC chamber are shown in f.

3.1. The receiving antenna's VSWRs should be similar. The figure h ... •• maximum, average, and minimum VSWR obtained by rotating the tuner t.nr . complete cycle at discrete frequencies from 100 MHz to 1 u GHz.

variations exist, especially at frequencies below 1 GHz.

frequencies the variations become comparatively negligible

open field VSWR of the antennas.

The coupling efficiency of the antennas placed insid* th

expressed in terms of a ratio of the net input power d'-li • transmitting antenna to the power available at the terminals ‘ tm

antenna. These ratios, referred to as chamber loss, ar* s

v

r

• The curves shown in the figure are the average, and minimum

5
using the mode-tuned approach, the minimum loss measured using the modestirred approach, and the minimum loss determined from rf pulse measurements.

The results were determined by rotating the tuner through a complete

revolution while recording the transmitted and received power. Impedance

mismatch between the power detector used to measure the received power and

the receiving antenna have not been accounted for in figure 3.2. As noted

from the data shown in figure 3.1. this can contribute to a significant

error, especially at frequencies below 1 GHz. The magnitude of this source

of error is discussed in [5] and is included in the error estimates given in

section 5 of this report.

3.2 Chamber Quality Factor

The chamber's quality factor (Q) influences the operation of the

chamber in a number of ways. Examples are tuner effectiveness, rf input

power requirements and the accuracy with which test field levels can be

established inside the chamber. Another important factor is the time

required for the chamber to charge up to a steady state condition after the

input signal is applied. This influences, of course, the chamber's response

characteristics for pulsed rf testing.

From resonant cavity theory the chamber's "composite Q" , (Q), can be

determined [9] from the equation

2 S6 1 + 3A ( 1 +

’

1 6 a b c

3 where V is the volume in m , S is the internal surface area in

f~2

/ is the skin depth in m, A is wavelength in m, and a, b,

/ toy o chamber's internal dimensions in m.

2 m , and 6 s and c are the

The chamber's composite Q is determined [9] by averaging the 1/Q values

of all possible modes within a small frequency interval about the frequency

of interest. Equation (1) is considered a maximum or upper bound because it assumes the chamber losses are due only to finite wall conductivity. In

reality, some loss will also occur due to leakage from the chamber, loss in

antenna support structures, and loss in the chamber's wall coatings.

An alternative means of determining the chamber Q can be achieved by

using data obtained for the chamber's loss. Chamber loss is determined

experimentally by measuring the difference between the net input power, , delivered to the chamber's transmitting antenna, and the power available, P , at the reference antenna terminals. If the energy is uniformly distributed

over the volume of the chamber, an empirical value, (Q') can be obtained [10]

using the equation

V P Q' = 16 tt —zr- . (2)

A^ t

6
Results obtained by using equation (1) to calculate the composite Q and

by using the chamber loss data shown in figure 3-2 and equation ( 2 ) to

calculate the experimental Q are shown in figure 3.3. Figure 3.3 (a) gives

the curves for the calculated values of Q and Q' for the RADC chamber and

figure 3.3 (b) shows the ratio of Q to Q'. At frequencies above

approximately 1 GHz the ratio approaches a constant value approximately equal

to 1.6 or a difference of approximately 2 dB. This indicates that at

frequencies where equations (1) and (2) are valid, there are relatively small

losses due to contributing sources other than the loss in the chamber walls.

At frequencies below approximately 1 GHz, the chamber mode density is

insufficient to obtain uniform distribution of the energy over the volume of

the chamber. This affects the accuracy with which the empirical value of Q,

Q' can be determined. Thus the ratio of Q to Q' increases as the frequency

decreases as shown in figure 3.3 (b).

3.3 Tuner Effectiveness

Another consideration in the operation of a rever berat ing chamber is

the effectiveness of the tuner to redistribute the energy in the chamber and

hence to obtain complete randomness in the characteristics of the test

signal. To achieve this the tuner must be electrically large and be shaped

or oriented to distribute energy equally well into all possible chamber

modes. A test to determine how well the tuner is functioning is to measure

the ratio of the maximum to minimum received power of the receiving antenm

as a function of tuner position. This is done while maintaining a constant

net input power to the chamber's transmitting antenna. A large ratio

indicates that the tuner is, in fact, redistributing the scattered fields

inside the chamber effectively. The results of these measurements are given

in figures 3-4 and 3.5. Figure 3.4 shows an example of the measured E-field

inside the chamber as a function of tuner position at three discrete

frequencies. Note how the number of maximum and minimum increase as tr

frequency increase. Figure 3.5 gives the ratio of the maximum to minimum

received power as a function of frequency obtained from data similar to that

shown in figure 3.4 over the frequency range 0.1 to 18 GHz. A numb- r of

factors, including those referred to earlier and related to the design of tn- tuner, can influence the magnitude of this ratio. For example, a reductior

in this ratio after placing rf absorber and/or an EUT inside the ch^mb^r : an indication of the loading effect or reduction of the chamber's qu •. factor (Q) caused by the absorber and/or the EUT. A minimum ratio of

is suggested for proper operation of the chamber [5].

3.4 Test Zone E-Field Uniformity

Tests were made to determine the E-field uniformity in the cn : • a function of spatial position and frequency. Ten liBS iso’r : [11,12,13] designed to operate at frequencies up to 2 GHz wcr* pi

the chamber as shown in figures 3.6 and 3.7. Eacr pro:

orthogonally oriented dipoles aligned with the chamber's ixcs.

were made of the field strength of each orthogonal com.

d

n- s* • •• locations for each tuner position (200 steps of 1.6 d s for : 1000 MHz and 400 steps of 0.9 deg for frequencies 1.0 . . were normalized for a net input power of 1 W applied at. r.h^ inpw

of the transmitting antennas. The maximum and av-'-rag

7
component and the vector sum (total) of the components were then determined

from the complete data sets. The results of these measurements are shown in

figures 3.8 (a) and (b). The spread of the data shows the spatial field

variation inside the enclosure at the indicated frequencies. The field

strength drops significantly at frequencies below approximately 300 MHz. This

is due to the low frequency response of the transmitting antenna used and to

the lower number of modes available in the chamber at the lower frequencies.

Higher density (closer frequency intervals) measurements were made between

100 and 150 MHz to improve the accuracy in determining the spatial field

variation at these lower frequencies. The results indicate the chamber is

operating properly, but, with rather large spatial variations at the lower

frequencies, down to 100 MHz. The spread in the data (spatial distribution)

at selected frequencies is summarized in table 3.1. The spread is as great

as ± 9 dB at 100 MHz decreasing to approximately ± 3.5 dB at 300 MHz, ± 2.5

dB at 1.0 GHz, and ± 2.0 dB at 2.0 GHz. The average values, determined

statistically from the data measured at the ten locations, for the average

and maximum E-fields of each component and their composite total are

summarized in figure 3-9. The relative amplitudes of the field components

are approximately the same and the composite total of the average E-field

components is approximately 9.8 dB or a ratio of /3~~ greater than the

individual components. This indicates that the measured values of the

average of each component are independent of polarization in the chamber.

Hence, the average field inside the chambers appears to be randomly

polarized. The composite total of the E-field components' maxima (figure 3-9

(b)) is less than 9.8 dB. This indicates that the maximum measured values

for each component are not independent ^ E

x (total) a f

' unction oE

E , ., etc.). This is similar to the results obtained in the NBS

y ( total

)

reverberating chamber and appears to be inherent in the reverberating chamber

measurement method. The implication is that if multiple receptors are

involved in establishing the maximum susceptibility of an EUT (for example in

measuring the E-field in the chamber by using an isotropic probe with 3 orthogonal dipoles), the difference between the maximum and average response

determined for the EUT may be incorrectly weighted (less than the 7-8 dB

anticipated). (See section 2.3 of [5] for the explanation why 7~8 dB

difference in signal amplitude between the maximum and average field

strengths is anticipated.)

3.5 E-Field Amplitude Calibration

The field strength in the chamber can be determined in two ways. The

first is to measure the power received by the reference antennas, and then

determine the equivalent power density in the enclosure using the equation

[5]

E a

9ttA (V/m)

,

(3)

where E is the equivalent electric field in V/m, n' is the statistically

3

averaged wave impedance of the chamber in q, P * is the equivalent power

2 density in W/m , in the enclosure, A is the wavelength in m, and P

' is the
average measured received power in W. The averaged wave impedance is assumed

to be approximately equal to 1 20tt fi. The validity of (3) has been verified

and is discussed in section 2.3-1 of NBS TN 1092 [5].

The maximum and average electric field strengths inside the R A DC

chamber determined from the receiving antenna power measurements and (3) are shown in figure 3.10. These data were obtained for 1 W net input power to

the chamber's transmitting antenna.

The electric field strength inside the chamber can be determined a second way by measuring it with one or more calibrated probes. Data obtained

using a 1 -cm dipole probe fabricated at the NBS are also shown on figure

3.10. The probe was calibrated in a planar field using a TEM cell [1*4] at

frequencies up to 500 MHz and in an anechoic chamber at frequencies from 500

MHz to 18 GHz [15]. The assumption is made that the field strength over the

aperture of the probe inside the reverberating chamber will approximate the

planar field used to calibrate the probe. This is reasonable, at least at

frequencies for which the probe is electrically small. Also, the open-space

far-field gain of an electrically small dipole is small (1.76 dB). Thus, the

probe-measured fields should be equivalent, within approximately 1.76 dB, to

the E-fields determined using a receiving antenna. This is true if the

equivalent gains for the probe and receiving antenna, after being placed

inside the chamber, are assumed to be 1. The agreement shown is typical of

the random variations in the data used to determine the field strength inside

reverberating chambers.

3.6 Comparison of R ADC Rever ber at i ng Chamber with NBS Anechoic ana

Reverberating Chambers

Comparisons of the response or EMS/V characteristics of EUT obtained

using an anechoic chamber and a reverberating chamber are typically made in

terms of peak values. The main reasons for this are that the EUT ' s worst

case performance or susceptibility is desired and the practical consideration

of the difficulty in obtaining a true average response for an EUT from

anechoic chamber data. Even determining the EUT's peak response in n. anechoic chamber (depending upon how well behaved the EUT receiving pat"' rn characteristic is) can require considerable effort involving complete patt

measurements

.

The value of performing these comparisons is to estimate a "corr- . a :

• factor" between results obtained in the r ever berat i ng and the an chambers. This should first be done for reference standard EUTs and th* n f EUT more typical of operational equipment. For this effort, two

EUT were used. These were (1) a 1 cm dipole probe antenna r- f earlier (data of figure 3-10) and (2) two ridged horn antennas (on- :• . .

• to operate in the frequency range, 0.8 - 12 GHz, and one design-: \ in the frequency range, 0.8 - 18 GHz). The measurements w - r • ; the RADC reverberating chamber and in the *4.9 m x 6.7 m x • . •. 22.0 ft x 27.9 ft) NBS anechoic chamber located in Boulder, or

A comparison of the peak output response data obtain- : f- . dipole is given in figure 3-11- The probe's output responn- in

chamber is greater, in general by an average of approxi mat <

: v

". output in the r ever ber at i ng chamber. This is as ex[ • •

• corresponds approximately to the gain of the electrical 1 y

9
measured inside the anechoic chamber. The suggested correlation factor

between EUT EMS/V (response) measurements is the free-space gain of the EUT

[5].

Comparison measurements of the peak response of a ridge-horn antenna

designed to operate in the frequency range, 0.8 - 12 GHz, were made using the

RADC reverberat ing and NBS anechoic chamber. The results are shown in figure

3.12. For the anechoic chamber measurements, the horn was bore-site aligned

with the transmitting antenna to obtain the peak response. The horn's

response (figure 3-12 (a)) is greater in the anechoic chamber than in the

r e ver ber at i ng chamber. The horn was calibrated to determine its free-space

gain, in dB, which was then subtracted from the anechoic chamber measured

response. These results are shown in figure 3.12 (b). The results agree, in

general, as expected.

Figure 3.13 shows the comparison of the peak response of the two

similar type ridged-horn antennas to the same level test field inside the

RADC rever ber at i ng chamber. The results shown are as expected since the

response of the antennas should be similiar. (The effective gain of both

antennas is 1 inside the rever berating chamber).

It is of value to compare evaluation results obtained for different

reverberating chambers, for example, the NBS and RADC reverberating chambers.

Such a comparison was made to answer two significant questions: First, can the input power requirements of a uncalibrated chamber be estimated from its

measured Q relative to a second calibrated chamber, and second, are

susceptibility test results obtained for the same EUT in different

reverberating chambers comparable?

Recall from theory that the power density inside a second chamber can

be estimated from a calibrated first chamber by using the equation [5],

where is the power

chamber (1 or 2 )

.

3 volume , = 38.19 m

V

2 Q, « <

i >

density and E is the electric

a If we assume the first chamber

and the second chamber is the

( 4 )

field for the particular

is the NBS chamber with a larger, RADC chamber with

3 a = 186.9*1 m , the ratio, V^/V^ , is 4.89. The ratio of the experimentally

determined Qs , Q

’ / Q T, for the two chambers is approximately 0.17. This gives

a calculated value for the ratio of E /E, of approximately -0.8 dB or the

a

i a2 RADC chamber should require approximately 0.8 dB less input power to generate

the same test field level as compared with the NBS chamber.

We might expect at first, because the NBS chamber is considerably

smaller than the RADC chamber, that the NBS chamber would require less power

to generate the same test field level than the RADC chamber. This is not so because of the difference in the Qs of the two chambers resulting from the

difference in the surface coating used inside the chambers. The RADC chamber
is coated with galvanized zinc over steel and the NBS chamber is coated with

nonconduct i ve epoxy over steel. The zinc surface, with its higher

conductivity than steel, results in a much higher Q for the R ADC chamber than

the NBS chamber. Results shown in figures 3-1^ compared with figure 3 - 1 0

verify these calculations and hence answer the first question affirmatively.

Figure 3.14 shows the fields inside the NBS chamber calculated from the

chamber's reference antenna received power measurements and also measured by

the calibrated 1 cm long dipole probe. These data are similar to the data

shown in figure 3.10, obtained for the RADC chamber. The net input power was normalized to one watt for both chambers. The fields inside the two chambers

(RADC and NBS) are approximately the same, consistent with the small (0.9 dB)

predicted difference. Thus the answer to the first question (Can the input

power requirements of a chamber relative to a second calibrated chamber be

estimated from its measured Q?) is yes.

Figure 3-15 shows the response of the NBS 1 cm dipole measured inside

the NBS rever berat i ng and anechoic chambers. These data can be compared with

figure 3-11 showing the same type data for the RADC chamber. Again the

agreement is well within the uncertainties estimated for the two measurement

techniques and different facilities. Thus the answer to the second question

(Are susceptibility test results obtained for the same EUT in differen - reverberating chambers comparable?) is also yes.

4.0 Reverberating Chamber Pulsed RF Evaluation Results

4 . 1 Background

Because of the significant potential of the rever berat i ng chamber

method for performing cw EMS/V testing of an EUT, there is considered,

interest in using this technique also for pulsed rf EMS/V testing. Work .. performed to evaluate the RADC chamber's response to pulsed rf excitation t determine the feasibility of using it in this mode of operation anc to

determine estimates for correction factors necessary to correlate results

obtained to a free-space environment.

Parameters of electromagnetic interference (EMI) signals tt r contribute to upset in electronic equipment include: (a) total energy,

peak amplitude, and (c) transient time characteristics . All these piare modified, relative to free space, for signals transmitted i ns

i

r e ver ber at i ng chamber. Their char acter i zat i on inside a reverb- r ••

. : ,• chamber, particularly for pulsed rf fields, provides information r •••. . .

• • determine correction factors as a function of the input pulse p-; r . also provides insight into the inherent limitations associate ; .

• this complex environment for pulsed rf EMS/V testing. re- factors are influenced by the Q factor of the chamber, . required for the pulsed wave's amplitude to rise to its st>- :y inside the chamber and to decay to zero after the input sign

a function of the chamber's Q. These "charge and decay" i by artificially lowering the chamber's Q, for examp e, by

amounts of rf absorber. However, this reduces the a r / the test field amplitude. Results of work to ev i u : character i sti cs of the chamber when excited by pu : :

• widths and frequencies, and with the chamber loanee wide . . absorber are contained in this section.
4.2 Evaluation of Pulsed RF Measurement Results

Measurements were made, using the mode-stirred approach, to determine

the effect of lowering the reverberating chamber's Q on the charge/decay time

of the rf pulsed field excited inside the chamber. Varying amounts of

pyramidal rf absorber were placed in the center of the chamber on top of a 15 cm high platform of plastic foam. Measurements were first made at 900 MHz

(the lowest test frequency) since data obtained previously, of the NBS

rever ber at i ng chamber's response to pulsed rf, indicated the response time

increases as the frequency decreases. Results of these measurements are given in figures 4.1 (a - i). Each figure shows two traces. The top trace

is the input pulse applied to the chamber's transmitting or excitation

antenna. The bottom trace is the time-domain signal received by the

chamber's reference receiving antenna but with the polarity reversed to

separate the two traces. The spread in the top traces (input signal) is due

to the variations in the input signal amplitude caused by movement of the

chamber's tuner. The magnitude of this variation is frequency dependant,

decreasing as the frequency increases as indicated in figure 4.2. Figure

4.1a was obtained with the chamber empty (no absorber). Figures 4.1b - 4. If are for increasing amount of absorber starting with a 1/2 piece (12" x 24")

of 3" thick rf absorber, ending with 4 pieces (24" x 24") of 24" thick rf

absorber. The influences on both the amplitude and response time are

obvious. Figures 4.2a - 4.2c show data similar to figures '4.1a - 4 . 1 i but at

selected frequencies from 0.9 GHz to 16 GHz. Data for three amounts of

absorber loading are given: with no absorber, with 1 piece of 5" x 24" x 24"

rf absorber and with 1 piece of 24" x 24" x 24" rf absorber. Again the

effect of absorber loading on both the amplitude and time-domain response is

apparent

.

It is interesting to compare the chamber's response time (charge or decay), obtained by calculation from the experimentally determined Q values,

to the measured response time. Recall from [8] that the response time is

approximately 2Q/w 0 for the signal to rise or fall (1-1/e) to 63 % of the

steady state amplitude. These results are shown in tables 4.1 and 4.2.

Table 4.1 gives the measured average chamber losses, the associated

calculated Q factors, and the calculated charge/decay times determined at 900

MHz for the varying amounts of absorber. These data were derived from the

results shown in figures 4.1 (a - i) normalized with the results of figures

4.3 sheets 1 -10. Table 4.2 gives the measured average chamber losses, the

associated calculated Q factors, the calculated charge/decay times, and the

measured charge/decay times at the selected frequencies for the chamber

empty, with 1 piece of 5" x 24" x 24" rf absorber, and with 1 piece of 24" x 24" x 24" rf absorber. The calculated and measured response times for the

chamber differ somewhat, but are still within the margin of error expected

for these type of measurements.

The mode-tuned measurement approach, as previously defined, was used to

determine: (a) chamber loss for rf pulse excitation compared to cw excitation

thus allowing a comparison of the total energy available in the test field

for cw or pulsed rf testing, and (b) the ratio of the received pulse

amplitude verses input pulse amplitude as a function of time after the input

pulse is turned on. (These are two of the three parameters that are

important in assessing potential upset due to EMI of EUT for EMS/V testing.)

Again, measurements were made with the chamber empty (no absorber) and with
the chamber loaded with 1 piece of 5" x 24" x 24" rf absorber and with 1 piece of 24" x 24" x 24" rf absorber. Measurements were made to determine

the ratio of energy received by the chamber's receiving antenna to the energy

transmitted by the transmitting antenna for pulsed rf signals. The total

energy received was determined by using numerical integration to determine

the energy (amplitude x time) of the received signal relative to the

transmitted pulse energy (amplitude x time) as a function of pulse width and

rf frequency. Sample results were shown in figure 3-2 for a transmitted

pulse width of 1 ys . This pulse width should have the maximum deviation from

cw signal chamber loss for rf pulse widths of interest, (1 - 10) ys since the

shorter the pulse width, the less time the chamber has to respond (charge

up). The cw and rf pulse chamber losses agree within 2 dB at all

frequencies. One can conclude for these measurements that even with

significant pulse dispersion as a function of time, as shown in figures 4.1

and 4.2, the energy associated with the field inside the chamber is

approximately the same as for cw testing. (The chamber loss, cw or rf pulse,

is approximately the same.)

Sheets 1-10 in figure 4.3 give the results of the mode-tuned received

pulse amplitude measurements data presented statistically as a function of

time. These graphs give an indication of the time required for the chamber

to charge up to its steady state amplitude and to decay to zero after the

pulse is turned off. The results indicate an obvious dependence on frequency

and the chamber's Q. Charge-up time results from the time required for the

input signal, radiated from the source antenna, to complete all significant

multiple reflections that contribute to the final field inside the chamber.

Each graph shows two curves, one for the maximum and one for the average

received signals. Smooth curves have been added to some graphs to provic

estimates for calculating ratios of the transmitted signal amplitude to the

received signal amplitude as a function of time, as referred to earlier. By

examining these data at the selected frequencies, the approximate charge or decay times can be determined as a function of frequency for the empty

chamber and for the chamber loaded with the two different size pieces of rf

absorber. These results are shown in

given in terms of the time required for

percent of the maximum amplitude. The

that the input pulse duration, at the

above the respective curves for the

greater than 63 or 90 percent of the maximum steady state or cw am pi it . The steady state amplitude is achieved if the input pulse width is suffici'-n*

for the chamber to charge up 100 percent, or to its maximum output f o

r’ given input pulse amplitude.

figures 4.4 and 4.5. The results ar the received signal to rise 63 and 9

implication of figures 4.4 and 4.5 is

frequencies of interest, should b^

pulsed rf test field amplitude to be

If the transmitted input pulse duration is not longer th m t’.- time of the chamber, an error will result in terms of establishing : /• • peak amplitude of the test signal in the chamber. An •sti- • correction factor to apply for this condition can be found by cal j’ van; •• ratio of the received signal amplitude to the transmitted sign-i ; .

• .. a function of time after turning on the transmitted pul

(sheets 1 - 3) gives these results, determined from the da: of ?

.• .r- sheets 1-10. The corrections shown are the ratio of the r- . :• amplitude as a function of time, after the input pulse is *

• , steady state amplitude of the signal. The data are ; r> o • *

• pulse durations as low as 0.2 ys . Note the significari -

: .

• correction obtained by loading the chamber with rf -.bs
is that the data shov/n in the graphs of figure 4.6 can be used to correct for

test field signal amplitude reductions due to insufficient transmitted pulse

duration times. This assumption needs to be experimentally verified by

comparing the measured responses of a well characterized EUT to cw fields and

to pulsed rf fields of varying pulse widths, at the same frequencies, using

both an anechoic chamber and an evaluated (calibrated) reverberating chamber.

Figure 4.7 is another way to display the data shown in figure 4.6. The

correction factors are shown for discrete input pulse widths as a function of

frequency rather than for discrete frequencies as a function of input pulse

width. The dots are the actual data extracted from figure 4.6 and the solid curves are the smooth approximations for this data.

5.0 Summary of CW Measurement Uncertainty

5.1 Estimate of Uncertainty in Establishing CW E-Field Amplitude Inside the

Chamber

As indicated in section 3, EMS/V test fields established inside the

chamber can be determined two ways: either using a reference receiving

antenna or a calibrated probe. If a reference receiving antenna is used, the

field is determined in terms of "equivalent" power density or "equivalent"

electric field strength by using (3). If a calibrated E-field probe is used,

the field strength is measured relative to an equivalent probe response in a known planar field. An estimate of the uncertainties in each of these

methods can be determined by analyzing the contributing parameters involved

in each method. The significant sources of error are summarized in tables

5.1

and 5.2 for the mode-tuned and mode-stirred approaches respectively,

within their appropriate frequency bands. Four major categories are

identified. The first is the uncertainty in determining the received power

measured by the reference antennas (la. tables 5.1 and 5.2), or in measuring

the E-field with the calibrated probe (1b. tables 5.1 and 5.2). The

uncertainty in determining the received power is broken up into five

components: cable loss, attenuator calibration, reference antenna

efficiency, power meter or spectrum analyzer measurement uncertainties, and

impedance mismatch. Values shown for the first four components are typical

of estimated uncertainties stated for these types of measurements and

instruments. The fifth component, impedance mismatch is the uncertainty in

determining the actual power delivered to the detector (load) attached to the

antenna (source) relative to the power available. The actual or measured

power is a function of the impedance match between the source and load, with

maximum power transfer occurring when a conjugate impedance match exists.

Power transfer between a source and a load is given as

P

f

fraction of maximum . ,

, i 2

. .

. .. , , , , (i- r )(i- r available power absorbed =

1 S1 1 L by the load , . „ „ . 2

I 1 - r s

r J

(5)

where r and r denote complex reflection

O Li load respectively. The magnitudes, | F^

appropriate VSWR by the expressions

coefficients for the source and

and | r | can be obtained from the

14
VSWR - 1 l S or L . ( 6 ) VSWR + 1

The VSWRs for the reference antennas (sources) and power detectors

(loads) used in the RADC rever berating chamber are given in table 5-3- These

values were used to calculate the estimated uncertainties shown for the

mismatch errors in tables 5.1 and 5.2. Both the statistical average and

maximum values are given.

Discussions of the uncertainty in calibrating E-field probes in planar

fields can be found in [1^,15]. Their response to fields insice a

r e ver ber at i on chamber has been shown to be less than their response in a planar field. The difference is proportional to their free-space gain [5].

Typical probes used are electrically short dipoles over most of the frequency

range. Sometimes, however, they are used beyond their resonance frequency

(for example the 1 cm dipole probe at frequencies above 15 GHz). The

corrections needed to correlate results obtained in the reverberating chamber

with those obtained in free-space or in anechoic chambers then correspond to

from 1.76 dB to 2.6 dB. The total estimated uncertainties of using the MBS

1 cm dipole probe to measure E-field amplitudes in the rever berating chamber

are shown on tables 5.1 and 5.2 (see 1b).

The second category of error, referred to as mixing or sampling

efficiency, is divided into two parts. The first part relates to the ability

to obtain a uniform spatial field distribution (statistically) inside the

chamber and to effectively destroy the polarization characteristics of the

exposure field. (The statistically determined response characteristics of

the EUT and chamber reference antenna are independent of their directional

properties.) The second part is the uncertainty due to limiting the number

of tuner positions per revolution when performing the measurement. This

source of uncertainty is different when determining the average as compared

to the maximum field as shown in the tables. Data contained in [5] (figur • 2.27 and table 6.4) were used in obtaining these estimates.

The third category of uncertainty relates only to determining th*

equivalent E-field strength in the chamber from the equivalent power dens it /. Recall that equation (3) assumes that the equivalent wave impedance i ns i :• the chamber is 1 20-ir SI. In reality this is not true as has been shown

However, data shown in [5] can be and were used to provide an estimate f >r this error. These data indicate that a wave impedance as great as 1 6C can exist at frequencies below 500 MHz when the maximum E field is m • sun

This corresponds to approximately 6 dB of correction. However, •. signif icarr

amount of data obtained to date, indicates that a well behaved r- (7 _ 9 dB difference) exists between the measured peak and average val j< : the E-field. This suggests, at least at frequencies above a f

• m

megahertz where the chambers are highly moded, that the peak v •. . wave impedance for the maximum measured E-field inside the chambers

as the frequency increases. In the limit, it should approach l - .. this source of error decreases as frequency increases. Thes- otr,

•

are reflected in the uncertainty estimates shown in the tables.

The fourth source of error occurs if corrections f or •

• variations due to the loading effect of the chamber on th" V:\v- f t.
antennas are not applied. These corrections are made when using the modetuned approach and hence are not included in table 5.1. They are not made

however, when using the mode stirred approach and hence are shown in table

5.2.

The total worst-case uncertainties for each method (receiving antenna

and calibrated probe) of determining the E-field for both the mode-tuned and

mode-stirred approaches are shown at the bottom of the appropriate table.

These uncertainties should be regarded as a conservative estimate. The

probability of the true value of amplitude of the test field being near an extreme is small. This is because the probability of all error sources being

at their extreme value in the worst possible combination is almost zero.

A more realistic method of combining uncertainties is the root-sum-ofthe-squares (RSS) method. The RSS uncertainty is based on the assumption

that the errors are statistically independent of each other and hence combine

like random variables.

Finding the RSS uncertainty requires that each individual uncertainty

be expressed in fractional form. The method of calculation follows the name

(that is, square the components, add those squares and then take the square

root). The results for both methods of determining the E-fields are shown at

the bottom of tables 5.1 and 5.2.

5.2 General Comments

Some general comments on interpreting uncertainties of immunity

measurement results based upon the above experimental error analysis are

appropr iate

.

1) The mismatch error at frequencies below 2 GHz, (particularly if

corrections are not made for the transmitting or receiving antennas

mismatches looking into their source or load), will cause the field

determination inside the chamber to be low. This also causes the EUT

response results to be lower than they actually are. For example, the

low frequency data of figures 3-10, 3-11* 3.1*1, and 3-15 should be

corrected (response increased) proportionally to the systematic offset

error estimates shown in tables 5.1 and 5.2.

2) The wave impedance, when the peak response of an EUT is measured,

appears to be higher than 1 20ir Q , especially for frequencies lower than

500 MHz. This means that if the free space wave impedance of 1 20tt ohms

is used in determining the corres pondi ng peak amplitude of the exposure

field, there will be a systematic offset error resulting in too low a calculated E-field exposure value. Since the actual E-field is higher

than the calculated value, this results in too high a EUT response

indication for a specified E-field exposure. If the E-field is

determined using a calibrated E-field probe, there still remains a degree of uncertainty since the wave impedance is different in the

calibration environment as compared to the r e ver ber at i ng chamber

environment. For this reason, this source of error was included in

calculating the total and RSS uncertainties in establishing the E-field

using the probe method.

16
The spatial variation in the measured, statistically determined E-field

in the chamber resulting from a complete revolution of the tuner(s)

decreases from as great as ± 9 dB at 100 MHz to less than ± 2 dB at 2.0

GHz. This variation should continue to decrease as the frequency

increases. However, high variations exist in the response data

obtained for the reference standard EUTs (1 cm dipole and ridged horn)

at frequencies where the spatial E-field variation are small. This is

due to the other contributing sources of error as discussed in 5.1. A way to reduce this problem is to increase the number of frequencies at

which data are taken (clustered around a frequency of interest) or increase the number of reference receiving antennas or probes used to

determine the exposure field and then average the data (for example as was done in figure 3.9).

Summary and Conclusions

The practical lower frequency limit recommended for using the RADC

chamber is approximately 150 MHz.

Spatial variations in the E-field maximum and average values determined

in the chamber test volume are shown in table 3.1. These data wer°

determined using the mode-tuned approach with 200 tuner increments : 100 to 1000 MHz and 900 tuner increments at 1000 to 2000 MHz for one complete tuner revolution. The limitation for determining the spatial

E-field variation is most likely due to the increasing mode density -a: hence field complexity in the chamber as a function of frequency,

limited sample size then becomes insufficient to determine the act . . maximums with greater accuracy. In reality, the spatial E-field

variations should continue to decrease (less than ± 2.0 dB) above 2 GHz

if sufficiently large data samples are taken and the measurer- a"

instrumentation has adequate dynamic range and precision.

Antennas used within the chamber for transmitting energy or for

determining the test E-field amplitude should not be used outside th ir

recommended frequency range.

The mode-tuned approach is recommended for use with the chamber • frequencies below 1 GHz. Either mode-tuned or mode-stirred car t

• from 1 to 2 GHz. The mode-stirred approach is recommended for us- :! 2.0 GHz. This allows for some overlap in the measurement appro

selected. Based upon antenna V SWR and EUT response data obt :i r.- : f the evaluation measurements, the following approaches and number of

samples per tuner (s) revolution are suggested for per f or - testing

.

Frequency Range Method # Tuner Positions

0.15 - 1.0 GHz Mode -Tuned 200

1.0 - 2.0 GHz Mode -Tuned 900

1.0 - 9.0 GHz Mode -Stirred > 3000

9.0 - 18.0 GHz Mode-Stirred > 5000

The maximum E-field is approximately 8 dB greater t- >n field established inside the chamber.
6. The theory for predicting input power requirements for a second unknown

chamber based upon the known requirements of a first chamber appears to

be valid.

7. Results of reference standard EUT response measurements made using the

RADC reverberating chamber are approximately the same as for the NBS

re ver ber at i ng chamber. Hence EMS/V data obtained for an EUT in

similarly operated, but different re ver ber at i ng chambers should be

approximately the same.

8. The " correlation factor" between free space (anechoic chamber) EMS/V

results and reverberating chamber obtained results appears to be the

free-space (far - field) gain of the EUT. This implies that

susceptibility criteria determined for an EUT using a r e ver ber at i ng

chamber must include an additional factor proportional to the EUT's

open-field estimated gain as a function of frequency.

9. Item 8 also implies that the directional character i st i cs of an antenna

or EUT placed inside a rever berati ng chamber are lost, resulting in

their equivalent gain of unity in this complex environment.

10. If the chamber is used for pulsed rf EMS/V testing and the width of the

input pulse to the chamber is not longer than the chamber's charge time, an error will result in terms of establishing a known peak amplitude of

the test signal. An estimate of the correction factors to apply for

this condition is found in figures 4.6 and 4.7.

11. Use of rf absorber significantly reduces the chamber's charge time and

can be used to improve the fidelity of the rf pulsed exposure field for

EMS/V testing. However, the amount of absorber should not add more than

approximately 6 dB loss to the chamber (reduce the reference antennas

received power by more than 6 dB) and/or it should not reduce the tuner

effectiveness below 20 dB.

12. Additional work is needed to experimentally verify the correction

factors shown in figures 4.6 and 4.7. This could be done by comparing

the measured responses of a well character i zed EUT to cw and pulsed rf

exposure fields as a function of frequency and pulse widths using both

an anechoic chamber and an evaluated (calibrated) rever berating chamber.

7.0 Acknowledgments

Work described in this report was sponsored by the RADC, Griff iss AFB,

Rome, New York, with Charles Blank as project monitor. The authors wish to

acknowledge his assistance in making this project possible. The authors also

wish to acknowledge the assistance of William Quinn, Rome Research

Corporation, Rome, New York, and John Workman and Darleen Agee, NBS for their

help and cooperation in performing the measurements discussed in this report

.

In addition, the authors express appreciation to Mark Ma, NBS, for his

support, helpful comments and editorial review.
8.0 Ref erences

[1] Mendes, H.A. A new approach to electromagnetic field-strength

measurements in shielded enclosures. Wescon Tech. Papers, Los Angeles,

CA. 1968 August.

[2] Cummings, J.R. Translational electromagnetic environment chamber, a new method for measuring radiated susceptibility and emissions. Proc.

IEEE Int . Symp. on EMC; 1975; San Antonio, TX

.

[3] Corona, P.; Latmiral , G.; Paolini, E. ; Piccioli, L. Performance of a reverberation enclosure for power measurements in the microwave range.

2nd Symp. Tech Exhibition on EMC; 1977; Montreux, Switzerland).

[4] Bean, J.L.; Hall, R.A. Electromagnetic susceptibility measurements

using a mode-stirred chamber. Proc. IEEE Int. Symp. on EMC; 1978;

Atlanta, GA.

[5] Crawford, M.L.; Koepke, G.H. Design, evaluation and use of a reverberation chamber for performing electromagnetic

s us ce pt i bi 1 i t y/ vul ner abi 1 i t y measurements. Nat. Bur. Stand. (U.S.)

Tech. Note 1092; 1986 April. 148 p.

[6] Ma , M.T.; Kanda , M.; Crawford, M.L.; Larsen, E.B. A review of

electromagnetic compati bil ity/ interf erence measurement methodologies.

Proc. of IEEE; Vol . 73, No. 3; 1985 March.

[7] Project Progress Report - Final Report FY86 " EMR test facilities

Evaluation of re ver ber at i on chamber method for performing EM

measurements", Prepared for RADC/RBCM, Griffiss AFB, NY. by NBS,

Boulder, CO. Report NO. SR-723~1 _ 87. 1987 January.

[8] Crawford, M.L.; Koepke, G.H. Preliminary evaluation of rever berat ion

chamber method for pulsed rf immunity testing. IEEE Int. Symp. on EMI;

1 986; San Diego , CA

.

[9] Liu, B.H.; Chang, D.C.; Ma , M.A. Eigenmodes and the composite quality

factor of a rever berating chamber. Nat. Bur. Stand. (U.S.) Tech 1 1066; 1 983 August

.

[10] Corona, P.; Latmiral, G.; Paolini, E. Performance and analysi rever berating enclosures with variable geometry. IEEE Trans. ; '

; EMC-22. 1980 February.

[11] Larsen, E.B.; Ries, F.X. Design and calibration of the NB

el ect r i c- f i el d monitor [EFM-5], 0.2 to 1000 MHz. Nat. j r. (U.S.) Tech. Note 1033; 1981 March.

[12] Kanda, M.; Driver, L.D. An isotropic electr i c- f i e 1 d prob< witresistive dipoles for broad-band use, 100 kHz to 18 GHz.

on MT&T. Vol. MTT~35 ( 2 ) ; 1 987 February.

[13] Bensema, W.D.; Reeve, G.R.; Koepke, G.H. A mult:/

field measurement system. Proc. IEEE IMTC/85 Conf; 1
[14] Crawford, M.L. Generation of standard EM fields for calibration of

power density meters: 20 kHz to 1000 MHz. Nat. Bur. Stand. (U.S.)

NBSIR 75 8011; 1975 January. 40 p.

[15] Bowman, R.R. Calibration techniques for electromagnetic hazard meters:

500 MHz to 20 GHz. Nat. Bur. Stand. (U.S.) NBSIR 75-805; 1 976 April.

33 P20
TOP VIEW

9.78 m

SHIELDED ENCLOSURE

SIDE VIEW

T

3 . G9 m

J.

Figure 2.1. Cross sectional views of RADC reverberating chamber show inf- placement of tuner, transmitting and receiving antennas, a: probe used to evaluate E-field amplitude inside the chamber.

21
22

Figure

2.2a.

Photograph

of

interior

of

RADC

reverberating

chamber

showing

tuner

mounted

from

ceiling

and

transmitting

antenna

mounted

on

tripod

in

northeast

corner

of

chamber.
23

Figure

2.

2b.

Photograph

of

interior

of

RADC

reverberating

chamber

shoving

receiving

antenna

mounted

on

tripod

in

northwest

corner

of
24

Figure

2.3.

Photograph

showing

NBS

equipment

used

in

performing

chamber
TUNER

CONTROL

25

Figure

2.

4.

Block

diagram

of

NBS

system

used

in

cv

evaluation

of

RADC

reverberating

chamber.
26

Figure

2.5.

Block

diagram

of

system

used

for

evaluating

pulsed

rf

response

characteristics

of

RADC

reverberating

chamber.
Figure 3. 1. Statistical representation of the composite VSWR of the

transmitting antennas used to launch the fields inside the RADC

reverberating chamber.

0.1 1 10

Frequency (GHz)

Figure 3.2. Coupling efficiency (minimum and average losses) between

transmitted and received powers measured at antenna's terminals

inside RADC reverberating chamber. Losses determined for (a) cw measurements using mode-tuned, (b) cw measurements uBing

mode-stirred and (c) from pulsed rf measurements.

27
Qcomp/Qexp

(Ratio)

Figure 3.3. Theoretical composite Q and experimental 0' determined for RADC

reverberating chamber. (a) Theoretical and experimental values

of Q as a function of frequency. (b) Ratio of the theoretical

composite Q to experimental O' as a function of frequency.

28
29

Figure

3.4.

Examples

of

E-field

inside

the

chamber

as

a

function

of

tuner

position

for

1

watt

net

input

power.

(a)

100

HHz,

(b)

150

HHz,

and

(c)

200

HHz.
Effect

i

veness

(dB)

Figure 3. 5. Ratio of Maximum to minimum received power obtained by rotating

tuner in the frequency range 100 MHz to 18 GHz inside the RADC

reverberating chamber.

30
9.78 m

TOP VIEW

ISOTROPIC PROBES 1-10 LOCRTED RS SHOWN

SIDE VIEW

T

3.69 m

A

Figure 3.6. Cross sectional views of RADC reverberating chamber showing

placement of NBS isotropic probes for evaluation of spatial

distribution of E-fields.

31
Figure 3.7. Photographs of interior of RADC reverberating chamber showing

placement of NBS isotropic probes for evaluation of spatial

distribution of E-fields.

32
(“>/a ap> pi*u-3

(“/A BP) Pt»U-3 ("A BP ) P l • > J-3

33

Figure

3.8.

Sheet

1

of

2.

Average

values

of

three

orthogonal

components

and

square

root

of

sum

of

squares

of

the

components

(Total).
Maximum

Tranaveraa

E-F Ia1d

Componanta,

RRDC

Chamber

Maximum

Vertical

E-F IaId

Componanta,

RRDC

Chamber

(“A BP) P t • I J

—3 T3Co

-pc0) coas . o ~ U "Ha

-i +> a o c t- o ~

O)o a x -p

-P

C P Qi O Co

a;a

a> e p o x u p

ai HH X O -P m «h

at o 3

•—t n o at ~ . T3 a -p c ~ -p « a> -

••

N OHS at a

XI o o -P s pau h

at UNO •H -H .p a r-| o a n p e sc

-P P tD o o

(0

C

H

rt • P ®

cn at oa » * soua i-i ® tj

«-• -p o 3 -H map o c at •h a

>ia -p p at o»

Ih z o » -H

-p

roa

tn

CD . c e a* -H 3 Ih a e a 3 -rlX prom

at E a x c E C a at X X -P u ~ ca

CD TJ cco> •HOC P -H

ffl -p

Ih » -P

at at -H X CD E pan at p c > at a

at > p p a h cpoX

•o

ato> o

COn

atp3

a>

(“A BP) P l ® I J

—3

(«uxA BP) P l • I J

—3

34
Field

(dB

V/m)

E-F

1

o1

d

(dB

V/m

)

Figure 3.9. Average values of the E-field strength measured inside RADC

reverberating chamber using array of 10 NBS isotropic probes with 1 vatt net input power: (a) average of the averages, (b) average of

the maximums. Transmitting antennas same as figure 3.8.

35
Field

(

dB

V/m

)

Figure 3. 10. Average and maximum E-field strengths determined inside RADC

reverberating chamber using: (1) reference antenna's received power

measurements (+), and (2) calibrated 1-cm dipole probe

measurements (*•**). Net input power normalized to 1 watt.

36
Probe

Output

(

dB

mV)

Figure 3. 11. Comparison of the peak responses of NBS 1-cm dipole probe to

normalized E-field of 37 dB V/m determined using RADC reverberating

chamber and NBS anechoic chamber.

37
Horn

Output

(dBm)

Horn

Output

(dBm)

Figure 3.12. Comparison of (1-12 GHz) ridged horn's peak responses to normalized

E-field of 37 dB V/m established inside RADC reverberating chamber

and inside NBS anechoic chamber. (a) antenna output versus

frequency, (b) Comparison of responses vith horn's free-space gain

subtracted from anechoic chamber results.

38
Horn

Output

(dBm)

Figure 3.13. Comparison of peak response of (1-18 GHz) and (1-12 GHz) ridged

horn antennas to 37 dB V/m normalized E-fields established inside

RADC reverberating chamber.

39
E-FIELD

MB

V/n

)

Figure 3. 14. Average and maxumum E-field strengths inside empty NBS

reverberating chamber for 1-W net input pover determined from: a)

reference antenna's received pover measurements, and b) calibrated

1-cm dipole probe measurements.

40
PROBE

OUTPUT

<dB

mV)

65

55

45

1 1—1

i nr 1

1—1

1111

/ \Au> #

****ANECHOIC -

CHAMBER

ooooREVERB —

CHAMBER

V -

! 1 1 - 1 1 J-J 1 1 1

1 1 1 1

35

25 -

.2

FREQUENCY (GHz)

Figure 3.15. Comparison of 1-cm dipole probe's peak response to EH field

established inside NBS reverberating and anechoic chambers. Output

normalized to E-field exposure of 37 dB V/m.

41
(a) No Absorber

1

1 -

ZZ *r

y

i - -

'2

1

.. r

5U§

5?

•

1 •.•0000 »«c 10.0000 utt c 20.0000 usic Funct lonl

Ch. 2

T 1 aebase Delta V Vaarkerl

50.00 »yolU/d(v

100.0 BVOltS'dlV

2.00 usec'div

-372. • avolts

160. • ivo 1

1

a

Offset

Offset

De 1 sy

0.000 volts 0.000 volts 0.00000 sec

Vaark er2 -212.0 >vo Its

(d) 1 Pc. 5* Absorber

Funct i onl

Ch. 2

T i sebsse Pelt* V Vssrkerl

50.00 svolts/dtv

50.00 avolts-'dfv 2.00 usec/div

-270.0 bvo

I

ts 160.0 bvo

I

ts

Offset

Offset

De I sy

0 . 000 volts

0. 000 volts 0.00000 sec VBBrker2 -110.0 BVO 1 t S

(k) 1/2 Pc. 3" Absorber

•.00000 sec 10.0000 usee 20.0000 usee

(©) ^ ^c * 0* Absorber

j 1

1 4- |

i T

I

1 tl

t !

'

I

i

©.60006 sec 10.

h

f

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Funct 1 onl 50.00 svolts'dlv Offset 0.000 volts

Ch. 2 « 50.00 svolts'dlv Offset 0.000 volts

T 1 sebsse 2.00 usee ^d i

v

De 1 sy 6.00000 sec Belts V -320.0 avolts Vssrker

1

160.0 bvo

I

ts Vsarker2 - -160.0 BVO 1 1 s

Funct i onl * 50.00 bvo 1 ts^di

v

Offset « 0 . 600 volts

Ch. 2 B 50.00 b vo 1 ts^dl

v

Offset 0.000 volts

T 1 aebase • 2.08 ijsec/di

v

De 1 sy 0.00000 sec Belts V -260.0 B VO 1 tS Vaarkerl • 160.0 BVO l tS Vaark er2 - -100.0 bvo Its

(c) 1 Pc. 3" Absorber (f) 1 Pc. 11* Absorber

Funct 1 onl

Ch. 2

T l BebBse Belts V VBorkerl

50.60 avolts'dlv

50.00 Bvolts/'dlv 2.00 usec'dlv

-265.6 bvo

I

ts 160.0 bvo ts

Offset

Offset

Be 1 By

0.000 volts 0.000 volts 6.00000 sec VBBrk er

2

-125.0 bvo 1 1

s

Funct 1 onl 56.00 bvo 1 ts'dl

v

Offset » 0.000 volts

Ch. 2 50.00 avolts'dl

v

Offset 6.000 volts

T 1 aebase 2.00 iusee /di

v

De 1 ay 0.00000 sec Delta V -250.0 BVO 1 t S Vaarkerl 160.0 BVO ItS Vaarker2 • -90.00 avolts

Figure 4.1. Sheet 1 of 2.

42
(g)

1 Pc. 24* Absorber — ---1.1,1-

'

1 i

i f

|

i

i

i I

+

j

!

j

i

! i

i

I

|

i

}

i

i

0.00000

i

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1

Belt* V Vasrkerl

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50. 00 avolts'div

2.00 usec^div

-247.0 avolts

160.0 evolts

Offset 0.000 volts

Offset « 0.000 volts

Belay - 0.00000 sec V»*rk er2 -87.00 •volts

(h) 2 Pcs. 24* Absorber

! I ! 1 i i ! i 0.00000 sec 10.0000 usee 20.0000 usee Funct t onl S0.00 BVOltS^dt V Offset a 0.000 volts

Ch. 2 20.00 • vo 1 1 s'di

v

Offset a 0.000 volts

T i sebue a 2.00 iusee ' d 1

v

Be 1 ay a 0.00000 sec felt* V a -223.2 •vo 1 tS Vaarkerl 160.0 •vol ts Va*rker2 -63.20 bvo its

(i) 4 Pcs. 24* Absorber

0.00000 see 10.0000 usee 20.0000 usee Function! 50.00 •volts^di

v

Offset - 0.000 volts

Ch. 2 - 20.00 •vol t s'd t

v

Offset a 0.000 volts Tlaebase 2.00 usec'dlv Belt* V -207.2 avolts

Belay 0.00000 sec Vaark er 1 160.0

Figure

•volts

4. 1. Sheet

Vairker2 •

2 of 2.

-47.20 •vol ts

Figure 4.1. Maximum received rf pulse waveforms inside RfiDC reverberat i r, r

, chamber determined with increasing amounts of rf absorber

loading at 0.9 GHz. Measurements were made using dual char

digitizing oscilloscope.

43
44

Figure

4.2.

Sheet 1

of

3,

(0.9,

1.3,

2.

0)GHz
9

GHz

2.9

GHz

555 * 1 oo» ; i

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s ss'sis:

iijjjil

i&iii

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555 t 5S2 2

45

Figure

4.2.

Sheet

2

of

3,

(2.9,

4.2,

5.

65)GHz
s ii: j i I is! § r

IU * I 222 s ;

ill mi

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11 : s 222 i
Racalvad

Pouar

(mllllHatta)

Raoalvad

Pouar

(milliwatt.)

Raoalvad

Powar

(Hitti)

no DC TUNED* 0.9 GHz v 20 us. No Rbsorbsr, Run 21

NODE TUNED* 2.0 GHz, 20 us. No Rbsorbsr, RUN 58

MODE TUNED* 1.3 GHz, 20 us. No Rbsorbsr, Run 29

MODE TUNED * 2.9 GHz, 20 us. No Rbsorbsr, RUN 37

Figure 4.3. Sheet 1 of 10.

amplitude is 1 W. and 5.65 GHz.

Input

at 0. 9 and

pulse width is 20 us. Input pulse

1.3 GHz and 0.1 W at 2.0, 2.9, 4.2

47
noDC TUNCD i 0.9 CH» , 9 ua, No Rbaorbar MODC TUNCD l 0.9 CHi, 10 ut. No Rbaorbar

NODC TUNCD i 0.9 GHs , 3 ua. 1 Ro . 9* Rbaorbar none TUNCDi 0.3 GH*. 19 ua. I Po. 9“ Rbaorbar

Figure 4.3. Sheet 2 of 10.

is 1 W. Frequency =0.9 GHz, input pulse amplitude

48
49

Figure

4.3.

Sheet 3

of

10.

Frequency

*1.3

GHz,

input

pulse

amplitude

is 1W

for

no

absorber

loading

and

0.1 W

for

absorb.-r

loading.
50

Figure

4.3.

Sheet

4

of

10.

Frequency

=2.0

GHz,

input

pulse

amplitude
51

Fxgure

4.3.

Sheet

5

of

10.

Frequency

=2.9

GHz,

input

pulse

amplitude
l l !) p»A|»o»d

:

f

I

Figure

4.3.

Sheet

6

of

10.

Frequency

=4.2

GHz,

input

pulse

amplitude

is

0.1

W.
53

Figure

4.3.

Sheet

7

of

10.

Frequency

=

5.65

GHz,

input

pulse

amplitude
3G0W

54

4.3.

Sheet 8

of

10.

Frequency

=8.9

GHz,

input

pulse

amplitude

is

0.1
TUNCD

It

55

Sheet 9

of

10.

Frequency =

12.0

GHz,

input

pulse

amplitude

is
56

Figure

4.3.

Statistical

amplitude

distribution

(maximum

and

average)

for

rf

pulse

waveforms

obtained

inside

RADC

reverberating

chamber

at

selected

frequencies,

input

pulse

widths

and

absorber

loadings.
Figure 4 . 4 . Time required for rf signal transmitted into RADC reverbera* ;r. ; chamber to rise 63X of steady state amplitude as a functior. cf

frequency. Chamber empty (no absorber), with 1 piece of 5* x 24 * x 24 * rf absorber, and with 1 piece of 24 * x 24" x 24* rf

absorber.

57
Charge

Time

(ms)

Figure 4. 5. Time required for rf signal transmitted into RADC reverberating

chamber to rise to 90% of steady state amplitude as a function

of frequency. Chamber empty, with 1 piece of 5" x 24" x 24* rf

absorber, and with 1 piece of 24" x 24" x 24" rf absorber.

58
Paisa Width (ua)

Figure 4.6. Sheet 1 of 3. Frequencies = 0.9, 1.3, and 2.0 GHz.

59
Figure 4.6. Sheet 2 of 3. Frequencies = 2.9, 4.2, and 5.65 GHz.

60
Figure 4.6. Estimated correction factor for amplitude response of teat

signal inside RADC reverberating chaaber as a function of rf

input pulse duration at selected frequencies.

61
Correction

Factor

(dB)

Figure 4.7. Sheet 1 of 3. RADC chamber empty (no absorber).

62
Correction

Factor

(dB)

Figure 4.7. Sheet 2 of 3. RADC chamber loaded with 1 piece 5* x 24* x 24*

rf absorber.

63
Correction

Factor

(dB)

rf absorber.

Figure 4.7. Estimated correction factor for amplitude response of test

signal inside RADC reverberating chamber as a function oi

frequency at selected rf input pulse durations. (Data

extracted from figure 4.6).

64
Table 3.1. Spatial variations in the E-field average and maximum values

measured inside the RADC reverberating chamber.

Frequency

(GHz)

Variation in E-field

(dB)

100

150

200

300

500

1000

2000

< ± 9.0

< ± 6.0

< ± 5. 0

< ± 3. 5

< ± 3.0

< ± 2. 5

< ± 2.0
Table 4.1. Influence of absorber loading on loss, quality factor and

charge/decay time of RADC reverberat ing chamber at 0.9 GHz.

Absorber

Loading

Chamber

Loss

(dB) Ave

Quality

Factor

Q

Charge/Decay

*Tirne

Us

none 14. 5 2B294 10.01

1/2 Pc. 3" 16.6 17445 6. 17

1 Pc. 3" IB. 5 11264 3. 9B

1 Pc. 5" 19. 1 9S1

1

3.47

1 Pc. B" 19. B 8350 2.95

1 Pc. 11" 20.3 7442 2.63

1 Pc. 24" 20.5 7107 2.51

2 Pc. (XI 22. 1 4917 1.74

4 Pc. 24" 23.5 3562 1.26

* Charge/decay time calculated from 2Q/uo and is based upon <l-l/e)A or 63 %

of amplitude of steady state field inside chamber after excitation source is

turned on or has decayed 63 % from steady state amplitude after source is

turned off.

66
Table 4.2 . Comparison of measured and calculated chamber charge/decay times

determined from chamber average loss measurements and from data of

figure 4.3. Chamber empty, with 1 Pc. 5" x 24" x 24" rf absorber,

and with 1 Pc. 24" x 24" x 24" rf absorber.

Frequency

(GHz)

Chamber Loss

(five) (dB>

Errmtv 5" 24"

Quality Factor

X 10000

Empty 5" 24"

Calculated

Charge/Decay

time (us)

Empty i 5" 24"

Measured

Charge/Decay

time ( us

)

EmDt v 5" 24"

0. 9 14.5 1 19. 1

: 20. 5

I 2. 83 0.98

1 0. 71 10. 0

1

1 3.47 2. 51 5. 62 2. 12 • 1.43

1.3 18. 0

1 22. 1 i 24. 0 3. 81 1. 48 0. 96

1 9. 33

1 1 3.63

l

i 2. 34 5. 19 1. 97 1. 35

2. 0 22. 0

' 26.2 1 27.8

1 5. 52 2. 10 1.45

1 8.79 .3.34 1 2. 31 4. 69 1.79 1. 24

2. 9 26.0

i 30.2

i

i 31.

8

6.70 2. 55

1 1.76 7.36

,

1 2.80 1.93

l 4.23 , 1. 63 1. 15

4. 2 29. 8 | 34. 1

i 35. 4 8.49 ro tH in

i 2. 34 6.43 ‘ 2. 39

i 1.77 3.80 •

i 1. 48 i 1.07

5.65 33.0 1 36.9 i 38.0 9. 89 '4.03

i

, 3. 13 5.57

. t

1 2.27

i i 1. 76 3.46 1

| 1.34 1. 00

8. 3 37.5

1 41.8

i

' 42. 9

I 13.7 ' 5. 10

l

! 3. 96 4.90 1. 82 '

i i 1.41 2.90 i

i 1. 16 0. 89

12.0 40.8 1 43. 1

| 44.8 15.7 1 9. 26

i

i 6. 26 4. 17 • 2.46 1

i 1.66 2.57 '

i 1.03 0.82

16.0 43.9

l

1 46. 1 ! 46. 7 18. 3

111.0 ' 9. 58 3. 63 ! 2. 19 I 1.91 2.24 ; 0. 90 0. 76

67
Table 5. 1. Summary and estimates of measurement uncertainties for determining

cv field strength inside RADC reverberating chamber.

Mode Tuned (100 MHz -2.0 GHz)

Error <dB)

Source of Error 100 MHz 150 MHz 200 MHz 500 MHz 1.0 GHz 2.0 GHz

Ave. Max. Ave. Max. Ave. Max. Ave. Max. Ave. Max. Ave. Max.

la. Received Power

Cable Loss ± 0. 05 ± 0.05 ± 0.05 ± 0.05 ± 0.05 ± 0. 10

Attenuator Cal. ± 0. 10 ± 0. 10 ± 0.10 ± 0. 10 ± 0.10 ± 0. 10

Antenna Efficiency ± 0. 05 ± 0.05 ± 0.05 ± 0.05 ± 0.05 ± 0.10

Pwr. Meter Cal. ± 0. 20 ± 0.20 ± 0.20 ± 0.20 ± 0.20 ± 0.20

Sub Total ± 0.40 ± 0.40 ± 0.40 ± 0.40 ± 0.40 ± 0.50

Mismatch -4. 1 -8. 2 -3.4 -8.2 -3.7 -7.4 -1.5 -4.1 -1.1 -2.8 -1.3 -2.4

lb. E-Field Meas.

1-cm dipole probe ±1.0

2. Sampling

Efficiency

Spatial Field Var. ±9.0 ±6.0 ±5.0 ±3.5 ±2.5 ±2.0

Limited Sample Size

(see [5]) ±0.2 ±0.5 ±0.2 ±0.5 ±0.2 ±0.5 ±0.2 ±0.5 ±0.2 ±1.5 ±0.3 ±1.0

Sub Total ±9.2 ±9.5 ±6.2 ±6.5 ±5.2 ±5.5 ±3.7 ±4.0 ±2.7 ±4.0 ±2.3 ±3.0

3. Wave Impedance -2. 8 -2. 8 -2.4 -2.4 -2.0 -2.0 -2.0 -2.0 -2.0 -2.0 -2.0 -2.0

(see 153) +2.0 +6.0 +2. 0 +6.

0

+ 2.0 +6.0 +2.0 +4.5 + 2.0 +3.0 + 2.0 +3.0

E-Field Determined

By Receivina Ant.

Total Worst -16.5-20.9 -12.4 18 -11 -15.3 -7.6 -10 -6.2 -9.2 -5.1 -7.9

Case Error +10.6+14.9 + 9.2+13.9 + 8.6 + 12.9 +5.6 +8.4 + 5.1 +7.4 + 4.3 +6.0

RSS Error -10.5-12.9 -7.5-10.7 -6.7 -9.4 -4.5 -6.1 -3.6 -5.3 -3.4 -4.4

+8. 5+10.4 +7.5 +9.6 +6.5 +8.9 + 3.8 +5.3 + 3.4 +5.0 +2.7 +3.9

E-Field Determined

By Dipole Probe

Total Worst -13.0-13.3 -9.6 -9.9 -8.2 -8.5 -6.7 -7.0 -5.7 -7.0 -5.3 -6.0

Case Error +12.2+16.5 +9.2+13.5 +8.2+12.5 +6.7 +9.5 + 5.7 +8.0 + 5.3 +7.0

RSS Error -9.7 -10 -6.7 -7.0 -5.7 -5.9 -4.3 -4.6 -3.5 -4.6 -3.2 -3.7

+9. 5+11.3 +6.6 +8.9 +5.7 +8.2 +4.3 +6.1 + 3.5 +5.1 + 3.2 +4.4

68
Table 5. 2. Summary and estimates of measurement uncertainties for determining

cw field strength inside RADC reverberating chamber.

Mode Stirred (1.0 GHz - 18.0 GHz).

Error (dB)

Source of Error 1.0 GHz 2.0 GHz 4.0 GHz 8.0 GHz 12.0 GHz 18.0 GHz

Ave. Max. Ave. Max. Ave. Max. Ave. Max. Ave. Max. Ave. Max.

la. Received Power

Cable Loss ± 0.05 ± 0.10 ± 0.10 ± 0. 15 ± 0. 15 ± 0.20

Atten. Calibration ± 0. 10 ± 0. 10 ± 0.15 ± 0. 15 ± 0.20 ± 0.20

Antenna Efficiency ± 0.05 ± 0. 10 ± 0.15 ± 0.15 ± 0.20 ± 0.20

Spec. Analyzer Cal. ± 1.00 ± 1.00 ± 1.50 ± 1.50 ± 1.50 ± 1.50

Sub Total ± 1.20 ± 1.30 ± 1.90 ± 1.95 ± 2.05 ± 2.10

Mismatch -1.3 -3.1 -1.3 -2.4 -0.8 -1.3 -0.4 -0.8 -0.5 -2.3

lb. E-Field Meas.

1-cm Dipole Probe ± 1.0 ± 1.0 ± 1.5 ± 1.5 ±2.0 ±2.0

2. Sampling

Efficiency

Spatial Field Var. ±2.5 ± 1.5 ± 1.0 ±0.5 ±0.5 ±0.5

Limited Sample Size

(see C 5 ]

)

±0.1 ±0.2 ±0. 1 ±0.3 ±0.2 ±0.5 ±0.3 ±0.7 ±0.3 ±1.0 ±0.3 ±1.5

Sub Total ±2.6 ±2.7 ±1.6 ±1.8 ±1.2 ±1.5 ±0.8 ±1.5 ±0.8 ±1.5 ±0.8 ±2.2

3. Wave Imped £ 120TT Average 4. ±2.0, -2.0 < Maximum < +3.0

(see [5])

4. Input Power Var. -1.3 -2.4

E-Field Determined

By Receivina Ant.

-1.2 -1.7 -0.8 -1.3 -1. 6 -0. 5 -1.0

Total Worst -8.4-11.4

Case Error +5.8 +6.9

-7.4 -9.2

+4.9 +6.1

-6.7 -8.0

+5.1 +6.4

-6.8 -7.9

+ 4. 8 +6.

5

-5.9 -6.6

+ 4.9 +6.6

-8.2 -9.6

6.9 +7.3

RSS Error -4.0 -5.3

+3.5 +4.2

E-Field Determined

By DiDole Probe

-3.4 -4.2

+2.9 +3.7

-3.2 -3.6

+2.9 +3.9

-3.3 -3.6

+ 2.9 +3.9

-3.1 -3.3

+ 3.0 +3.9

-3.9 -4.4

3.0 +4.3

Total Worst -6.9 -8.9

Case Error +5. 6 +6. 7

-5.8 -6.5

+ 4.6 +5.8

-5.5 -6.3

+ 4.7 +6.0

-5.9 -6.6

+ 4.3 +6.0

-5.3 -6.0

4.8 +6.5

-5.8 -7.2

4.8 +7.2

RSS Error -3.7 -4.3

+3.4 +4.2

-3.0 -3.3

+ 2.8 +3.6

-2.9 -3.2

+ 2.8 +3.7

-3. 1 -3.3

+ 2.6 +3.7

-3. 0 -3. 2

2.9 +3.9

-3.1 -3.7

2.9 *4.2

69
Table 5. 3. Estimates of upper limit of impedance mismatch uncertainties for

reference antenna's received power measurements for RADC

reverberating chamber.

VSWR Mismatch Error

Frequency Source Load (dB)

GHz Ave Max Max Ave Max

0. 1 7. 5 22. 0 1. 10 -4. 14 -8. 17

0. 15 6.0 22.0 1. 10 -3. 40 -8. 17

0. 2 6.5 18. 0 1. 10 -3.66 -7. 37

0. 3 5.0 15.0' 1. 10 -2.83 -6.67

0. 5 3. 0 7.5 1. 10 -1. 46 -4. 14

1. 0 2.5 5. 0 1. 10 -1. 07 -2. 83

1.0 2.5 5.0 1. 20 -1. 25 -3. 10

2.0 2. 5 4.0 1. 20 -1.25 -2. 44

4. 0 2. 0 2. 5 1. 20 -0. 81 -1. 25

8.0 1. 5 2. 0 1. 20 -0. 37 -0. 81

12.0 1. 5 1. 5 1. 30 -0. 48 -0. 48

18. 0 3.0 3. 0 1. 50 -2. 25 -2. 25

70
NBS-114A l REV. 2-80

U.s. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR 87-3080 December 1987

4 . TITLE AND SUBTITLE

EMR Test Facilities Evaluation of Reverberating Chamber

Located at RADC, Griffiss AFB, Rome, New York

5. authoR(S) Myron Crawford, Galen H. Koepke, and John M. Ladbury

6. PERFORMING ORGANIZATION (If joint or other than N BS, see instructions) 7. Contract/Grant No.

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP

)

10.

SUPPLEMENTARY NOTES

_ Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

hi bl iography or literature survey, mention it here)

This report describes measurement procedures and results obtained from evaluating the

reverberating chamber facility located at the Rome Air Development Center (RADC), Griffiss Air

Force Base, Rome, New York. The facility waB developed by the RADC for use in measuring and

analyzing the electromagnetic susceptibility/vulnerability (EMS/V) of weapon systems and the

shielding effectiveness of enclosures and shielding materials. A brief description of the

facility, including the instrumentation used for performing its evaluation and calibration by

the National Bureau of Standards (NBS), is given. Measurements described include: (1)

evaluation of the chamber's transmitting and receiving antennas' voltage standing wave ratios;

(2) measurement of the chamber's insertion loss or coupling efficiency versus frequency; (3)

measurement of the chamber's tuner effectiveness; (4) determination of the E-field uniformity

in the chamber's test zones versus frequency; (5) determination of the absolute amplitude

calibration of the test E-fields in the chamber based upon the reference antenna's received

power measurements and calibrated dipole probe antenna measurements; (6) comparison of

reference standard equipment under test (EUT) responses to test fields established inside the

RADC reverberating chamber and the NBS anechoic chamber; and (7) evaluation of the

performance characteristics of the reverberating chamber excited by pulsed rf at selected

discrete frequencies as a function of pulse width (0.2 - 20 Us) and the chamber's quality

factor (Q). Conclusions indicate that the chamber can be used at frequencies down to approximately 150 MHz for cw testing, and for rf pulsed immunity testing with pulse widths as short as 0. 3 Us by using rf absorber loading. Immunity testing to pulsed rf fields however

has some inherent limitations that are discussed in the report. Estimates of the cw measurement uncertainties derived empirically from the test results are given.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only prope names; and s orate key word'. : .

,

Key words: electromagnetic radiated susceptibility/vulnerability measurements; rever t it • : chamber.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

|y | Unlimited

| | For Official Distribution. Do Not Release to NTIS 80 ~| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. IS. Price

jX]] Order From National Techn ical Information Service (NTIS), Springfield, VA. 22161

. jlCOUM'OC

U S. GOVERNMENT PRINTING OFFICE: 1988-574-318 85.062 REGION NO 8
*